Outline

Background: At the Midwest Proton Radiotherapy Institute (MPRI), dose is deposited via a uniform scanning system with dose layer stacking. The spread out Bragg peak (SOBP) is generated as plates of an absorbing material are inserted into the beam, attenuating the range in steps of either 3 mm or 6 mm water equivalence. Due to breathing of a patient during irradiation, several tumor sites move during treatment. The interplay effects of the relative movement may introduce clinically significant errors.

Material and methods: Static percentage depth dose profiles of pristine Bragg peaks and SOBPs were obtained by inserting GafchromicÂ® EBT2 films in a water phantom as well as by performing multi layer ionization chamber (MLIC) measurements. A MatlabÂ® simulation, which sums the weighted static pristine proton beams according to the temporal behavior of the treatment nozzle and the tumor motion, was used to model the dose deposition out of pristine peaks and therefore predict SOBP dose distributions with and without motion.

Tumor motion was simulated by a LEGOÂ® mindstorms robot, which moved the films in the water phantom with a symmetrical, cosine-square like breathing trajectory.

Results: Dose distributions of a proton beam with a nominal range in water of R=16 cm and a modulation width of M=10 cm yielded significant dose deviations in the moving target. Dose deviations at the distal fall-off, at the entrance of the SOBP, and within the SOBP region were found. For a single 2 Gy fraction with a breathing amplitude of A= 7.5 mm and a breathing period of T=3.6 s, the observed dose deviations in the plateau region were up to 4.3%. Simulations for 30 fractions showed a destructive interference of the dose escalations, assuming a random starting point of the breathing phase.

Conclusion: For a single fraction of 2 Gy, deviations in the SOBP region of up to 4.3% could be found for simulated breathing patterns of clinical relevance. Additional investigations are needed for fractionation schemes with large doses per fraction and different breathing patterns.